This issue is partly addressed in the video 'PDF publishing for pro print': https://affinity.serif.com/en-gb/tutorials/publisher/desktop/video/337466797. Publications are presented to the printer as a set of individual pages, not spreads. During export  to a PDF file creating bleed for central gutter areas on both pages (of 2-page spreads) is automatically taken care of. However, this requires overlapping graphics and not images or vector objects that are placed exactly at the center gutter.